The present invention relates to the art of metal surface treatment. More specifically, it relates to the art of post-treating a conversion-coated metal surface to impart improved corrosion-resistance thereto.
Numerous techniques have been developed for protecting metal surfaces against corrosion. Of these techniques, the most common is phosphatization of the metal surface. This is accomplished by contacting the metal surface with an aqueous solution containing phosphate ions so as to form on the surface a corrosion-resistant, non-reactive phosphate complex coating. Depending on the type of metal to be treated, the phosphate baths may be based upon an alkali metal and/or ammonium phosphate or, on the other hand, may be a coating metal phosphate solution containing a coating metal ion such as zinc. Further corrosion-resistance is obtained by conventional means when the phosphatized surface is post-treated with a dilute aqueous acidic solution containing a chromium compound, usually a hexavalent chromium solution. Chromium and molybdate and complex oxide treatments are also employed.
The use of chromium-containing post-treatments, while satisfactory from a corrosion standpoint, has become environmentally objectionable. Accordingly, it would be desirable to develop a process for the post-treatment of a phosphatized metal surface which provides good corrosion resistance and paint adhesion characteristics without employing objectionable heavy metal compounds such as chromium.